This invention relates to data transmission systems and, more particularly, to a data receiver for a data transmission system employing the interleaved binary phase shift keyed modulation technique.
Interleaved binary phase shift keyed modulation is a particular form of the widely used binary phase shift keyed (BPSK) modulation technique which has been described in many publications. Reference may be made, for instance, to the book titled, Information Transmission, Modulation and Noise, by M. Schwartz, section 4-2, McGraw-Hill, New York, 1970, for a description of BPSK modulation, and to the books titled Data Transmission, by R. W. Bennett and J. R. Davey, chapter 10, McGraw-Hill, New York, 1965, and Principles of Data Transmission, by R. W. Lucky, J. Salz and E. J. Weldon, Jr., chapter 3, McGraw-Hill, New York, 1968, for a general description of the phase shift keyed modulation technique. Briefly, BPSK modulation is a technique wherein the bits to be transmitted are sent one at a time at instants which have a T-second spacing and are called signaling instants, by making each bit value correspond to one of two possible carrier phase changes relative to the carrier phase at the preceding signaling instant. The absolute carrier phase can assume, at each signaling instant, one of two possible values. By indicating the two possible values of the phase of the carrier in a so-called signal space diagram, a pair of points that are 180.degree. apart in phase are obtained which illustrate the modulation scheme employed.
In the interleaved BPSK modulation technique, the bit to be transmitted is made to correspond, at an even signaling instant, to a selected one of a first pair of points that are 180.degree. apart in phase in the diagram, this first pair of points illustrating a first modulation scheme, and, at an odd signaling instant, to a selected one of a second pair of points that are 180.degree. apart in phase, this second pair of points illustrating a second modulation scheme, with the first and second pairs of points being 90.degree. apart in phase. The only difference between BPSK modulation technique and interleaved BPSK modulation technique is that the latter uses alternatively two modulation schemes which are interleaved in time.
An advantage of interleaved BPSK modulation over BPSK modulation is that it provides a modulated signal, the envelope of which varies but little in time. Consequently, most of the energy remains concentrated in a well-defined, narrow frequency band in a nonlinear channel. Interleaved BPSK modulation has been advantageously used in satellite communication systems wherein signals are received and transmitted over several adjacent frequency bands through a single nonlinear transponder. Thus, it is essential to ensure that the method employed minimizes interferences between bands. However, there is an inherent disadvantage in this type of modulation. As has been mentioned above, interleaved BPSK modulation involves the use of two different pairs of points in the signal space diagram; and if the transmitted data are to be correctly detected, the receiver must know at any given signaling instant whether it is to use the first or the second pair of points as a detection reference. In other words, the receiver must be synchronized with the transmitter. In the early transmission systems that used interleaved BPSK modulation, a synchronization sequence was transmitted to enable detection before the data message. This was, of course, a time-consuming solution which required the use of additional equipment in both the transmitter and the receiver and of special resynchronization procedures in the event of loss of the phase of the signals that define the interleaving rate.